Refrigeration defrost by compressor motor



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REFRIGERATION DEFROST BY COMPRESSOR MOTOR Filed Jan. 13. 1969 1 I; 220460611 208 7 5 5""; 118 O 1 -H 96 98 v 250 1% @g .fzmes [1145065 ATTORNEY nited States US. Cl. 62156 2 Claims ABSTRACT OF THE DISCLOSURE An automatic defrost system for a domestic refrigerator having a hermetically sealed motor compressor unit in sealed, series refrigerant flow relationship with a condenser, expansion means, and an evaporator. When a predetermined frost buildup accumulates on the evaporator, means including a thermostatically responsive switch disconnects the compressor motor field winding terminals from a normal household line voltage and connects them to a secondary coil of a step transformer for reducing field winding voltage to a level that precludes motor rotation and compressor operation. Sufiicient electrical resistance heat is produced by the sealed winding to cause the sealed refrigerant to increase in pressure and temperature during the defrost period and reverse flow into the low pressure evaporator for removing frost buildup therefrom.

This invention relates to automatically controlled apparatus for defrosting domestic refrigerators and, more particularly, to such systems wherein electrical resistance heating is utilized to melt frost buildup from the refrigera. tor evaporator.

The accumulation of frost upon the evaporator of domestic refrigerators is a problem because it decreases heat transfer between the refrigerant in the evaporator and a refrigerator compartment being cooled thereby. This necessarily decreases refrigerator efiiciency and increases the operating cost. Frost buildup upon the evaporator interferes with the passage of recirculated air therethrough which transfers heat from the refrigerator compartment to the refrigerant within the evaporator.

In a refrigerator having a wrap-around or bonnet type evaporator (not illustrated), frost buildup in certain cases can decrease storage space available within the refrigerator compartment and may interfere with the removal of ice trays and like articles when they are in direct thermal contact with the evaporator.

In hot gas defrost systems valving means are included to reverse refrigerant flow through the system so that hot refrigerant gas passes directly to the evaporator rather than the condenser of the system. The refrigerants latent heat of condensation is thus transferred to the evaporator to effect defrosting. While suitable for their intended purpose, such arrangements are relatively expensive for use in low cost domestic refrigeration units.

Other prior art automatic defrost systems use electric resistance wires attached directly to the evaporator. The wires are energized to melt frost from the evaporator during the defrost cycle of operation. This defrosting method, of course, requires molding or other attachment of resistance wires to the part to be heated. From a cost standpoint it would be desirable to eliminate the need for such wires and their installation.

An object of the subject invention is to eliminate the need for auxiliary electrical heaters in automatic refrigeration defrosting systems by apparatus which energizes the compressor motor winding at a predetermined voltage level to cause resistance heating thereby suflicient to heat and pressurize adjacent refrigerant which flows atent into an evaporator for removing frost buildup therefrom. The predetermined voltage level precludes motor operation that is normally incident to a refrigeration cycle whereby the compressor is inoperative during such defrost refrigerant flow.

A further object of the subject invention is the provision of a refrigerator defrost system which converts the compressor motor winding into a resistance heating element for warming adjacent refrigerant and which directs the warm refrigerant through normal fluid carrying tubes from a hermetically sealed compressor and motor shell to the evaporator.

The above objects and others that eliminate the need for hot gas or auxiliary heaters during the defrost cycle of operation in a domestic refrigerator are attained in the present invention by utilizing the compressor motor winding, itself, as an electrical resistance heater during a defrost cycle and to do so by energizing the motor winding at a reduced voltage which precludes motor rotation and compressor operation. The motor winding energization is sufficient to heat and pressurize adjacent refrigerant within a shell hermetically Sealing the compressor and motor winding. The increased pressure of the warmed refrigerant within the shell surrounding the motor and compressor initiates fluid flow of hot refrigerant directly to the evaporator in a direction opposite to normal flow during the refrigeration cycle thereby to melt frost buildup from the evaporator.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

FIG. 1 is a perspective view partially in section showing a refrigerator cabinet containing necessary components of the subject invention;

FIG. 2 is a vertical cross-sectional view taken along the section line 22 of FIG. 1 of a hermetically sealed refrigerant compressor and electric drive motor assembly suitable for use in practicing the present invention;

FIG. 3 is a schematic drawing showing a refrigeration circuit associated with the assembly of FIG. 2 for cooling the refrigerator of FIG. 1; and

FIG. 4 is an electrical circuit diagram showing defrost control means for automatically controlling the operation of the circuit in FIG. 3.

In FIG. 1 of the drawings, a refrigerator cabinet 10 is shown enclosing a cooling compartment 12 and freezing compartment 14 for the storage of food therein. Doors 16 and 18 close and seal openings in the cabinet 10 to the cooling space 12 and the freezer space 14, respectively.

In the illustrated embodiment shown in FIG. 1 the compartments 12, 14 are cooled by a forced draft of air across a tube-fin type evaporator 20 located below a false bottom in freezer compartment 14 and in an insulated, horizontal partition 22 separating freezer compartment 14 and cooling compartment 12. More particularly, a shrouded fan 24- draws air from the evaporator 20 for discharge through a duct 26 into compartment 14 to porduce the air flow pattern shown by the arrows in FIG. 1. Return louvers 28 permit air flow from compartment 14 back to the evaporator 20.

Fan 24 also discharges air through a depending duct 30 into compartment 12 to produce a cooling air flow pattern shown by the arrows in FIG. 1. A return opening 32 in partition 22 communicates compartment 12 with the inlet side of the evaporator 20. This arrangement of evaporator, fan and duct work maintains a temperature of approximately 34 F. within cooling compartment 12 and a temperature range of to F. in the freezing compartment 14.

An electric motor driven compressor assembly 34 is supported within the lower portion of cabinet 10, and a condenser 36 is attached to the back surface of cabinet 10.

A schematic drawing of the refrigerator fluid circuit is clearly shown in FIG. 3 as including a compressor 38 driven by an attached electric motor 40. Compressor 38 and electric motor 40 are located Within and hermetically sealed by an outer shell 42.

During a normal refrigeration cycle compressed gaseous refrigerant from compressor 38 is discharged through a tube 44 to condenser 3-6 where its heat of compression is dispersed to atmosphere. From the condenser 36 the cooled compressed refrigerant flows through tube 46 to an expansion valve 48 from which the expanded refrigerant flows into evaporator to absorb heat from air returned from the cooling compartment 12 and the freezer compartment 14 within the refrigerator cabinet 10. From evaporator 20 refrigerant flows into an accumulator 50 in which liquid refrigerant is separated from vaporous refrigerant before being allowed to pass through tube 52 into the interior of shell 42. Refrigerant in the interior of shell 42 is drawn into compressor 38 by an intake valve (not shown) to be compressed and recirculated.

The cooling air drawn through evaporator 20 by fan 24 depositsmoisture upon the fins and tubes of evaporator 20. This moisture quickly forms frost upon the evaporator 20 restricting the heat transfer between the recirculating cooling air and the evaporator. The accumulation of frost upon evaporator 20 can eventually block the flow of air between the fins and tubes of the evaporator. This reduction in heat transfer and air flow results in inadequate cooling and wasteful additional operation of the refrigeration system.

The problem of frost accumulation upon the evaporator 20 is solved by utilizing the motor operated compressor assembly 34 as a heater. In FIG. 2 of the drawings the compressor and motor assembly 34 is shown in detail. The hermetically sealed shell 42 encircles and encloses an electrical motor 40 and attached compressor 38. The motor and compressor are supported within shell 42 by a plurality of springs 54 (only one of which is shown) interposed between inwardly turned brackets 56 attached to shell 42 and base portions 58 formed on the exterior of motor 40. Compressor 38 basically utilizes an eccentrically mounted impeller (not shown) rotated by a motor shaft 60 within a hollow interior cylinder chamber of compressor 38. A rotating axially offset portion 62 of shaft 60 extends through the compressor impeller and causes the impeller to move in the cylinder chamber. The motion of the impeller draws refrigerant into the interior of compressor 38-, compresses the refrigerant, and expels it through tube 44.

The electrical motor 40 has a cup-shaped main casting 64 with a concentrically formed bearing boss 66 which partially supports shaft 60. The compressor 38 and an end plate 68 are secured to the bottom end face of casting 64 by a plurality of cap screws 70 (only one of which is shown). A concentrically located bore within end plate 68 provides radial support for a reduced diameter bottom end 72 of shaft 60. An inwardly directed indentation 74 formed in the top end of shell 42 limits axial upward movement of the shaft 60. A hollow cylindrical motor stator 76 is secured to the upper end surface of the casting 64 by a plurality of screws '78 extending from an annular bearing plate 80 into casting 64. A movable rotor 82 is concentrically located within the hollow interior of stator 76 and is concentrically secured around motor shaft 60 for rotative movement therewith. Energization of electrical conductors within stator 76 creates a fluctuating magnetic field which is the motive force that causes the rotor 82 and shaft 60 to rotate and drive the compressor impeller.

The exposed position of stator 76 within shell 42 is of particular importance in practicing the present invention. Electrical current flowing through stator 76 generates heat energy which tends to heat the adjacent gaseous refrigerant within the hermetically sealed shell 42. Thus, because a good heat transfer relationship between the stator 76 and refrigerant within the shell 42 is essential to the practice of the present invention, the motor is in no way separated from the refrigerant.

Also of importance in the practice of the present invention is the arrangement of tube 44 and tube 52 in relation to the electric motor driven compressor assembly 34. Tube 44 which leads to condenser 36 is directly connected to the outlet of compressor 38 and passes through the wall of shell 42. Thus, tube 44 carries compressed refrigerant directly from the compressor 38 to the condenser 36. Tube 52 communicates the evaporator 20 with the interior of shell 42. Thus, tube 52 carries low pressure refrigerant into the interior of the shell. Because of this arrangement, the refrigerant within the shell 42 is at a relatively low pressure during the normal refrigeration cycle of operation. This type of electric motor driven compressor assembly suitable for the present invention is known as a low-side unit (for low pressure within the shell).

To control and obtain refrigerating action the motor driven compressor assembly 34 is energized to compress refrigerant which is drawn from the interior of shell 42 into compressor 38. The refrigerant is pumped through tube 44, condenser 36, tube 46, expansion valve 48, evaporator 20, accumulator 50, and tube 52 back into the interior of shell 42. More particularly, the compressor motor energization circuit is shown in FIG. 4 as extending from L through a conductor 84, a thermostatic switch means 86, a conductor 88, through an electric motor starter relay 90, a conductor 92, a run winding 94 of motor 40, a conductor 96, across a normally closed motor deactivation switch 98, and through a. conductor to L2.

To recirculate cooling air during the refrigeration cycle of operation a fan energization circuit extends from L through conductor 84, thermostatic switch 86, conductor 88, a conductor 102, fan 24, a conductor 104, across a normally closed fan deactivation switch 106, through a conductor 108 and conductor 100 to L The thermostatic switch means 86 is of conventional design such as the well known expansible bellows type or the bimetallic strip type. Thermostat 86 senses the temperature within the cooling compartment 12 to activate or deactivate the motor energization circuit so as to maintain the temperature within compartment 12 within a predetermined range.

The motor starter relay designated generally by numeral 90 includes a bimetallic overload contact 110 which, though normally in a closed position, will open and disconnect the motor energization circuit upon sensing a predetermined maximum current load. An electrical heater 112 biases the bimetallic overload contact arm 110 open under abnormal current load during normal refrigeration cycle operation hence securing the function of an overload protector in the motor energization circuit.

During the refrigeration cycle of operation, the motor energization circuit runs across a normally closed run winding switch 114 within the motor starter relay 90. Because electric compressor motors normally have insuflicient torque at low speed to start motor rotation, the motor starter relay 90 includes means to impart additional starting torque to the motor during initial energization. More particularly, relay 90 includes a normally open start winding switch 116 which is closed by a solenoid 118 momentarily to energize a start winding 120 of motor 40 through a conductor 122. Solenoid 118 which is serially connected with the run winding 94 of motor '40 moves the start winding switch 116 to a closed position and moves the run winding switch 114 to an open position when an initial surge of current passes through the solenoid caused by the closing of thermostatic switch means 86. This initial current surge is only momentary because the opposing electromotive force buildup within motor 40 rapidly counterbalances this current surge. The equalizing of current through the motor energization circuit quickly allows the run winding switch 114 to close and the start winding switch 116 to open. Motor 40 is then operating at its predetermined rotational speed.

As explained earlier a problem in domestic refrigerators is the accumulation of frost upon the evaporator 20. The circuit in FIG. 4 further includes a low voltage source represented as a step transformer 124 to supply electric current through the run winding 94 of motor 40 so that resistance heat therefrom may be utilized to melt frost which accumulates upon the evaporator 20. The primary coil 126 of transformer 124 is energized by a circuit which extends from L through conductor 84, a conductor 128, primary coil 126, a conductor 130, across a transformer energization switch 132, through a conductor 134 and conductor 100 to L The transformer 124 also includes a secondary coil 136 which produces a flow of current at a lesser than line voltage through a circuit extending from coil 136 through a conductor 138, conductor 92, run winding 94, conductor 96, conductor 140, across a heater energization switch 142, through a conductor 144 back to secondary coil 136.

The motor deactivation switch 98, the fan deactivation switch 106, the heater energization switch 142, and the transformer energization switch 132 are regulated by thermostatic means including a heater energization bellows 146 and an ambient temperature sensing bellows 148. Bellows 146 is fluidly connected to a capillary tube generally shown in FIG. 4 by the numeral 150. As can be better seen in FIG. 3, capillary tube 150 includes a schematically illustrated spiral coil 152 and a schematically illustrated coil 154. The capillary tube 150 and bellows 146 are filled with a thermally responsive fluid so that the bellows 146 contracts in response to a drop in temperature of the accumulator 50 below a predetermined temperature, thus signaling frost accumulation upon evaporator 20. This contraction of bellows 146 moves the motor deactivation switch 98 and the fan deactivation switch 106 to an open position and moves the transformer energization switch 132 and the heater energization switch 142 to a closed position. With the aforementioned switches in this position, the refrigeration system is in a defrosting cycle of operation. Fluid filled coil 154 is fluidly serially connected between bellows 146 and spiral coil 152 and is positioned in the flow of recirculating cooling air immediately before the evaporator. Coil 154 assures that the defrost cycle will be of sufiicient duration to remove frost accumulation on the evaporator 20. Coil 154 basically acts as a time delay means which prevents the rapidly warmed and expanding fluid in spiral coil 152 from terminating the defrost cycle of action before complete defrosting is accomplished.

The ambient temperature sensing bellows 148 has a bulb portion schematically numbered 156 in FIG. 4 which is filled with thermally responsive fluid and located on the outside of cabinet 10. Bellows 148 senses the ambient temperature in which the refrigeration system operates and modifies the frequency of defrost action to compensate for this factor. The structure and function of the defrost control apparatus which includes heater energization bellows 146 and ambient temperature sensing bellows 148 is more particularly described in US. Pat. 3,359,750 to Hanson, it being fully understood that the structural details of the defrost control apparatus form no part of the present invention.

When the heater energization bellows 146 opens the motor deactivation switch 98 and the fan deactivation switch 106 and closes the transformer energization switch 132 and the heater energization switch 142 in response to decreasing temperature in accumulator 50, the run winding 94 of motor 40 will be energized by the secondary coil 136 of transformer 124. The relationship of the number of coils of primary coil 126 to the number of coils of secondary coil 136 of transformer 124 is such that the secondary coil output voltage is insufficient to cause motor rotation. Thus, thermal radiation from the energized winding 94 causes refrigerant within shell 42 to increase substantially in temperature and pressure While the motor 40 remains in a static state. Because the lower pressure evaporator 20 is directly communicated by tube 52 to the interior of shell 42, flow therebetween of refrigerant may proceed in either direction in response to the relative pressure of evaporator 20 and the interior of shell 42. A greater pressure within the interior of shell 42 caused by the heating of winding 94 motivates warmed refrigerant to move through tube 52 into accumulator 50 and evaporator 20. This high pressure Warmed refrigerant causes frost accumulated on the evaporator 20 and accumulator 50 to melt.

It has been observed that an average sized evaporator, similar to the one illustrated in FIG. 1, requires approximately 650 watts of energy for 10 minutes for defrost purposes. For convenience, a normal household line voltage of 120 volts is selected in FIG. 4 across L and L for motor operation and transformer energization during the refrigeration cycle and the defrost cycle, respectively. To preclude motor rotation during the defrost cycle of operation, the voltage output of the transformer secondary coil 136 must be less than the motors minimum voltage requirement for rotation. It is contemplated that a horsepower electric motor be used to drive the compressor shown in the proposed embodiment of the invention. This motor is identical to that used in the 1968 Model FPD-121TN Frigidaire refrigerator and represents a fairly typical plural Wound induction electric motor requiring a minimum input of volts for motor rotation. The resistance of run winding 94 for this motor is approximately 4 /3 ohms.

The aforementioned 650 watt power input, necessary for defrosting an evaporator, represents the PR power output required of run winding 94. The following Equation A represents this resistance power output P of the run winding when energized by electrical current. Equation B is the familiar Ohms law by which I can be stated in terms of voltage V and resistance R, and substituted in Equation A to derive the following Equation C from which the required voltage input of run winding 94 can be determined.

Substitution of 4% ohms for R and 650 watts for P in Equation C determines the required voltage input of run winding 94 for defrost purposes. Thus, an input of 53 volts applied across winding 94 will suflice to heat the refrigerant for defrosting the evaporator. Since this voltage level is below the minimum voltage level needed for motor rotation (85 volts), rotor 82 and shaft 60 will remain motionless during this defrost cycle of operation. Warmed refrigerant will be forced by the aforementioned pressure buildup within shell 42 to flow through conduit 52 to evaporator 20 to thus effect defrosting action.

While the embodiment of the present invention as herein described constitutes a preferred form, it is to be understood that other forms might be adapted.

What is claimed is as follows:

1. In an automatic defrost system for a domestic refrigerator the combination of refrigerant circuit means including an evaporator subject to the accumulation of frost thereon; a compressor for circulating the refrigerant in said circuit means; a motor having an electrical wind? ing therein normally connected to a first voltage power source; a hermetically sealed shell surrounding both said motor and said compressor containing refrigerant in heat transfer relationship to said electrical winding of the motor; fluid communicating means between said evaporator and said shell; control means responsive to a predetermined air temperature within said domestic refrigerator to periodically energize said electrical winding at the first voltage power level to cause motor rotation and compressor operation; means including a second thermostatic switch responsive to temperature of refrigerant in said evaporator to alternately energize said electrical winding at a second voltage power level Whenever the temperature of the refrigerant within said evaporator falls below a predetermined minimum value; said second voltage level producing electrical resistance heat from said electrical winding while precluding field strength sufficient to cause motor rotation whereby refrigerant in said shell is warmed and directed through said fluid communicating means to said evaporator.

2. In an automatic defrost system for a domestic refrigerator the combination of: refrigerant circuit means including an evaporator subject to the accumulation of frost thereon; a compressor for passing refrigerant through said circuit means; a motor having an electrical winding therein normally connected toa first voltage power level to effect motor rotation; a hermetically sealed shell surrounding both said motor and said compressor containing refrigerant in heat transfer relationship to said electrical motor winding; fluid communicating means between said evaporator and the interior of said shell; thermostatic switch means responsive to the air temperature Within said domestic refrigerator to periodically energize said motor winding at said first voltage power level to effect motor rotation and compressor operation; circuit means including a second thermostatic switch bypassing said first thermostatic switch means and energizing said motor winding at a second voltage power level whenever the temperature of refrigerant within said evaporator falls below a predetermined minimum value; said second voltage power level producing electrical resistance heat from said motor winding while precluding field strength sufficient to cause motor rotation and whereby refrigerant in said shell is warmed and directed through said fluid communicating means to said evaporator for defrosting.

References Cited UNITED STATES PATENTS 3,133,429 5/1964 Grifiin 62469X 3,201,676 '8/ 1965 Fuhrman 62469X 3,208,23 7 9/ 196-5 Gerteis 62469X MEYER PERLIN, Primary Examiner U.S. Cl. X.R. 62277, 469 

